The zinc endopeptidase meprin (EC 3.4.24.18) is expressed in brush border membranes of intestine and kidney tubules, intestinal leukocytes, and certain cancer cells, suggesting a role in epithelial differentiation and cell migration. Here we show by RT-PCR and immunoblotting that meprin is also expressed in human skin. As visualized by immunohistochemistry, the two meprin subunits are localized in separate cell layers of the human epidermis. Meprin alpha is expressed in the stratum basale, whereas meprin beta is found in cells of the stratum granulosum just beneath the stratum corneum. In hyperproliferative epidermis such as in psoriasis vulgaris, meprin alpha showed a marked shift of expression from the basal to the uppermost layers of the epidermis. The expression patterns suggest distinct functions for the two subunits in skin. This assumption is supported by diverse effects of recombinant meprin alpha and beta on human adult low-calcium high-temperature keratinocytes. Here, beta induced a dramatic change in cell morphology and reduced the cell number, indicating a function in terminal differentiation, whereas meprin alpha did not affect cell viability, and may play a role in basal keratinocyte proliferation.

Procollagen C-proteinase (PCP) removes the C-terminal pro-peptides of procollagens and also processes other matrix proteins. The major splice form of the PCP is termed BMP1 (bone morphogenetic protein 1). Active BMP1 is composed of an astacin-like protease domain, three CUB (complement, sea urchin Uegf, BMP1) domains and one EGF-like domain. Here we compare the recombinant human full-length BMP1 with its isolated proteolytic domain to further unravel the functional influence of the CUB and EGF domains. We show that the protease domain alone cleaves truncated procollagen VII within the short telopeptide region into fragments of similar size as the full-length enzyme does. However, unlike full-length BMP1, the protease domain does not stop at this point, but degrades its substrate completely. Moreover, the protease domain cleaves other matrix proteins such as fibronectin, collagen I and collagen IV, which are left intact by the full-length enzyme. In addition, we show for the first time that thrombospondin-1 is differently cleaved by both BMP1 and its catalytic domain. In summary, our data support the concept that the C-terminal domains of BMP1 are important for substrate recognition and for controlling and restricting its proteolytic activity via exosite binding.

The procollagen C-proteinase (PCP) is a zinc peptidase of the astacin family and the metzincin superfamily. The enzyme removes the C-terminal propeptides of fibrillar procollagens and activates other matrix proteins. Besides its catalytic protease domain, the procollagen C-proteinase contains several C-terminal CUB modules (named after complement factors C1r and C1s, the sea urchin UEGF protein, and BMP-1) and EGF-like domains. The two major splice forms of the C-proteinase differ in their overall domain composition. The longer variant, termed mammalian tolloid (mTld, i.e., PCP-2), has the protease-CUB1-CUB2-EGF1-CUB3-EGF2-CUB4-CUB5 composition, whereas the shorter form termed bone morphogenetic protein 1 (BMP-1, i.e., PCP-1) ends after the CUB3 domain. Two related genes encode proteases similar to mTld in humans and have been termed mammalian tolloid like-1 and -2 (mTll-1 and mTll-2, respectively). For mTll-1, it has been shown that it has C-proteinase activity. We demonstrate that recombinant EGF1-CUB3, CUB3, CUB3-EGF2, EGF2-CUB4, and CUB4-CUB5 modules of the procollagen C-proteinase can be expressed in bacteria and adopt a functional antiparallel beta-sheet conformation. As shown by surface plasmon resonance analysis, the modules bind to procollagen I in a 1:1 stoichiometry with dissociation constants (K(D)) ranging from 622.0 to 1.0 nM. Their binding to mature collagen I is weaker by at least 1 order of magnitude. Constructs containing EGF domains bind more strongly than those consisting of CUB domains only. This suggests that a combination of CUB and EGF domains serves as the minimal functional unit. The binding affinities of the EGF-containing modules for procollagen increase in the order EGF1-CUB3< CUB3-EGF2 < EGF2-CUB4. In the context of the full length PCP, this implies that a given module has an affinity that continues to increase the more C-terminally the module is located within the PCP. The tightest binding module, EGF2-CUB4 (K(D) = 1.0 nM), is only present in mTld, which might provide a quantitative explanation for the different efficiencies of BMP-1 and mTld in procollagen C-proteinase activity.